Characteristics and adsorption of Cr(VI) of biochar pyrolyzed from landfill leachate sludge

Abstract

Landfill leachate sludge (LLS), originated from the infiltration of rainwater through dumping sites of waste and water contained in landfills, is a specific type of sewage. Since its large amount, environmental unfriendliness and compositional instability, the sustainable management and disposal of LLS requires urgent attention. In this study, LLS disposal problem was addressed along with the removal of chromium (Cr) from water. Firstly, the thermal behaviors of LLS were investigated through TG analysis. Secondly, LLS-based biochar (SC) was prepared in a tubular furnace, and the effect of pyrolysis temperature on the yield and properties of SC was investigated. Finally, using SC900 as Cr(VI) adsorbent, the influence of various factors on Cr(VI) adsorption were investigated. BET, FTIR, and XPS analyses were performed to determine the characteristics of SC and the variation of SC before and after adsorption. Results showed that, TG curve of LLS exhibited a rapid mass loss of 54.15% between 220 and 525 °C due to the cleavage of C-O and C-C bonds with the release of large amounts of CO2. The SC yields were in the range of 10.81–22.16% when the pyrolysis temperature ranged from 400° to 900°C. The optimal pyrolysis temperature for the preparation of the SCs was determined to be 900 °C, and SC900 had a relatively high BET surface area of 83.87 m2/g. exhibited the highest Cr(VI) adsorption of 17.46 mg/g. The main oxygen-containing functional groups (-OH, C-O, CO) present in the SC played an important role to adsorbed Cr(VI) and reduced it to Cr(III) at a low pH. As a result, 76.5–85.3% of the adsorbed Cr(VI) was reduced to the less toxic Cr(III). Moreover, only 0.52% (1.56 ppm) of the adsorbed Cr (from a 300 ppm solution) leached from SC900, which indicated the high stability of the adsorbed Cr in the SC.